\documentclass[11pt,letterpaper]{article}

\makeatletter
\newcommand\ifmember[2]{%
\in@{,#1,}{,#2,}%
\ifin@
\expandafter\@firstoftwo
\else
\expandafter\@secondoftwo
\fi
}
\makeatother

\begin{document}

\ifmember{3}{3,4}{True}{False}
\newcommand{\foo}{3,4}
\ifmember{3}{\foo}{True}{False}

\end{document}
